A traditional clutch functions as a releasable coupling between an engine and a transmission. Engines and transmissions are utilized in a number of diverse applications, including motor vehicles as well as stationary power plants. A simple friction clutch consists of two discs which are forced together by a spring when the clutch is engaged allowing the clutch to transmit torque from the engine to the transmission. Typically, at least one of the discs is lined with a material having a high coefficient of friction and able to withstand high temperatures without excessive wear. Alternatively, a double-faced friction disc may be interposed between two elements of the driving disc.
To engage the clutch, the driving disc is moved into contact with the driven disc thereby reducing the relative speed difference between the discs until they are coupled through the frictional force. As is well known, this process abrades the friction lining of the clutch thereby reducing its thickness. Therefore, on a subsequent clutch engagement, the driving disc must travel an incrementally greater distance for proper engagement. The clutch actuating mechanism is connected to the driving disc, so it must also change position with wear of the friction lining. Eventually, the driving disc travel will be beyond the limits provided for by the clutch actuating mechanism. The mechanism must then be adjusted to account for this wearing of the friction lining. An adjustment is also required after the friction material is replaced (due to excessive wear) to achieve the proper travel distance.
Thus, for consistent clutch operation, a constant travel distance between the disengaged position and the engaged position of the driving disc should be maintained. This requires adjusting the position of the clutch actuating mechanism as the friction material wears. This adjustment may be accomplished manually at periodic intervals, or automatically as friction material wear occurs. It is desirable to have an automatic adjustment mechanism although it often requires a more complex actuating mechanism or clutch which is also more expensive.
When the transmission is in neutral gear and the clutch is disengaged after being engaged with the engine running, the inertia of the driven members tends to keep them rotating, with only a gradual decrease in speed due to friction. The inertia is proportional to the mass of the rotating members and their equivalent radius of gyration. This inertia is especially significant in heavy-duty powertrain applications, such as those utilized by tractor semi-trailer vehicles, as well as stationary powertrains such as those used in power generation facilities and in oil drilling applications. Such demanding applications require more massive components having a correspondingly larger inertia to accommodate their increased torque requirements.
The residual rotation after the clutch is disengaged creates a delay of several seconds before the driven members slow down to the correct speed to allow engagement of the next transmission gear. Although this time period may seem trivial, it becomes significant in a heavily loaded vehicle ascending an incline where the loss in momentum during the delay may result in a missed shift. It also becomes significant when considering its cumulative effect in that many heavy-duty applications require the operator to shift fifteen times or more before reaching highway speeds. Therefore, after the clutch is fully disengaged, it is desirable to reduce the time required for the driven members to slow down, in the case of a moving upshift, or to stop rotating completely, in the case of a stationary shift.
The device used to reduce the time required to slow down or stop rotation of the driven members after the clutch is fully disengaged is generically referred to as an input shaft brake. It can also be called an inertia brake, a clutch brake, or an upshift brake. A simple type of brake is normally used under low-torque conditions, such as when the engine is at idle and the vehicle is stopped, to engage the starting gear. An example of this type of brake consists of a single steel plate having a friction material on one or both faces and splined to the input shaft of the transmission. Upon disengagement of the master clutch, further travel of the non-rotating master clutch release bearing or similar actuating device clamps the plate with friction material against the transmission housing thereby decelerating the input shaft.
Traditionally, two arrangements of release bearings have been utilized. In the first arrangement, the release bearing moves rearward to disengage the master clutch while in the second arrangement, the release bearing moves forward to disengage the master clutch. Heavy-duty commercial vehicles, such as tractor semi-trailer trucks, typically utilize the first arrangement which then allows the input shaft brake to be actuated by further travel of the release bearing. The second arrangement may also be utilized with an input shaft brake, however, the actuating mechanisms for the master clutch and the input shaft brake would need to travel in opposite directions.
A higher torque capacity brake may be used to improve the speed of upshifts. Typically, a small, multiple-disc clutch is used to perform this function. This kind of brake may be mounted in-line with the transmission input shaft or may be off-axis. The in-line arrangement may be directly actuated upon disengagement of the master clutch by further travel of the master clutch release bearing. The off-axis arrangement may be actuated by fluid pressure, such as hydraulic or pneumatic pressure, and may be connected to the input shaft through a primary drive gear of the transmission rather than directly.
The vehicle operator indicates the desire to utilize the input shaft brake by depressing the clutch pedal so that it travels beyond a detent position. The detent position corresponds to the point of complete disengagement of the master clutch. The fully depressed clutch pedal actuates a pressure source (as described above) which is used to force the multiple friction plates of the input shaft brake together thereby applying the input shaft brake and slowing or stopping the inertial rotation.
It is important to coordinate the actuation of the input shaft brake with the complete disengagement of the master clutch. If the input shaft brake is applied prior to complete disengagement, the torque being transmitted through the master clutch may cause excessive heating and wear in the input shaft brake resulting in premature failure. A delay in actuation of the input shaft brake after the master clutch is fully disengaged is undesirable since it defeats the purpose of using an input shaft brake in the first place. The vehicle operator will also have trouble controlling application of the input shaft brake if it does not always engage at the same position of the clutch pedal.
As is well known in the art, coordinated control of the master clutch and the input shaft brake can be accomplished by manually coupling the actuators for these mechanisms. However, the change in master clutch travel resulting from friction material wear also changes the relationship between the disengagement of the master clutch and the application of the input shaft brake (input shaft brake wear is normally insignificant). It is desirable to maintain a constant relationship between these two events for consistent application of the input shaft brake which requires adjustment of the input shaft brake actuator, the master clutch actuator, or both.